This invention relates to electro-optic displacement measuring apparatus and systems and more particularly to new and improved fiber optic displacement measuring apparatus and systems having increased working distance and measuring range capability.
Fiber optic displacement devices utilizing bifurcated fiber optic bundles in combination with a light source and a photo detector, or other suitable light receiver, for measuring displacements are known in the prior art. A typical device is illustrated schematically in FIG. 1 and is described in U.S. Pat. No. 3,327,584 which is assigned to the assignee of the present invention. Such devices comprise a fiber optic bundle divided at one end into separate arms. One arm contains transmitting fibers and the other arm contains receiving fibers. The other or common end comprises a common bundle of transmitting and receiving fibers. When placed with face of the common end of the fiber optic bundle in close proximity to a target, light from the transmitting fibers impinges on the target and is reflected back to the receiving fibers where it is conducted to a photo sensor or other light receiver. The amount of reflected light is related to the distance between the end face of the fibers in the common bundle and the surface of the target, the diameter of the fibers, the numerical aperture of the fibers, the geometrical distribution of the transmit and receive fibers, the total number of fibers, and the reflectivity of the target surface. Thus, starting from the position of direct contact of the end face of the fiber optic bundle with the surface of the target as the gap between the bundle end and the target increases, greater amounts of light impinge on the receiving fibers in a substantially linear relationship to the displacement of the target. While such prior art arrangements are effective, they suffer from several undesirable limitations. For example, in order to achieve the maximum sensitivity and linearity possible, the working distance (i.e. the distance from the common end to the target) is restricted to the order of 1-5 mils. Greater working distances produce significantly reduced sensitivity and linearity while lesser working distances become difficult to achieve and maintain to the required tolerance. Another limitation is that the range of displacements measurable with a desired high accuracy is limited to about 1-3 mils. Yet another limitation of the prior art devices is that the target be at least as large as the common end of the fiber optic bundle.